As is known in the prior art, the systems of regulation of the machine-direction basis weight of the paper produced by means of paper machines operate as follows. The flow of the thick stock entering into the wire pit is regulated by means of a basis weight valve based on the measurement of the basis weight in the dry end of the paper machine. The basis weight of the paper web is measured by means of measurement detectors traversing in its cross direction, and the result of measurement of the cross-direction basis weight profile is produced as an average value and passed into the system of regulation as a feedback signal. From the basis weight valve, the flow of thick stock is passed, in a manner known in the prior art, into the wire pit and into which wire pit, the wire waters are also passed from the wire part of the paper machine. In the wire pit, the thick stock flow and the wire waters are mixed together, and the diluted stock flow thus obtained is passed, in a manner in itself known, through stock cleaning and deaeration devices into the inlet header in the headbox and from there further through the distributor manifold, possible stilling chamber and the turbulence generator of the headbox into a slice duct of the headbox. Out of the slice duct, the stock suspension jet is discharged onto a forming wire or into a forming gap defined between a pair of forming wires.
In the prior art, the cross-direction basis weight profile of the paper produced by means of paper machines is often regulated by means of profiling of the height of the slice opening based on the measurement of basis weight taking place in the dry end of the paper machine and described above. In recent years, what is called dilution regulations have also become more common, in which a dilution medium, typically wire water or clear filtrate or, in exceptional cases, a stock more dilute than the stock in the headbox, is fed into discrete feed points separate in the cross direction in connection with the headbox. By means of this dilution water feed system, the cross-direction basis weight profile of the slice jet is profiled together with, or without, regulation of the top slice bar. It is a particular advantage of the dilution regulation that the headbox can be run with a slice opening at a uniform height, so that the cross-direction flows in and after the slice jet, arising from the profiling of the height of the slice opening, and the distortions in the fiber orientation profile in the paper, as a result of such cross-direction flows, can be substantially avoided.
It is the most pronounced drawback of the prior art systems of regulation that monitor the machine-direction basis weight profile of paper that the response time of the system of regulation is very long. The delay in these systems of regulation from the actuator to the basis weight measurement point consists of the delay caused by the stock flow from the wire pit to the headbox and of the time of the passage of the paper web from the headbox to the measurement frame in the dry end of the machine. The magnitude of the delay depends on the stock flow velocity in the pipe system of the short circulation and on the machine speed.
The dead time in this type of system of regulation is typically of an order of one minute. Further, the regulation is made slower by the fact that, after a change in the position of the basis weight valve, reaching of a stable situation requires equalization of the consistencies in the entire short circulation. The time constant arising from this in the process is typically several dozens of seconds.
The long dead time and the time constant restrict the operation of this system of regulation as follows:
In a stable running situation, the regulation is capable of eliminating variations of very long cycles only. As a rough upper limit are considered disturbances of a cycle of 100 seconds, but in practice even this estimate is in most cases clearly excessively optimistic. PA1 In situations of change in the basis weight, such as change of paper grade, reaching a new stable situation takes a long time. PA1 Optimal tuning of the regulation is difficult, because the dead time varies in compliance with the process conditions, and it cannot be predicted precisely by means of computing in different situations.